Terrestrial Network

Description

In 3GPP terminology, a Terrestrial Network (TN) refers to the entire ecosystem of standardized cellular communication systems whose network nodes (base stations, core functions) and user equipment are located on the Earth's surface or within the atmosphere. This encompasses all generations of cellular technology standardized by 3GPP, including GSM (2G), UMTS (3G), LTE (4G), and NR (5G). The architecture of a TN is fundamentally hierarchical and geographically distributed, consisting of a Radio Access Network (RAN) and a Core Network (CN). The RAN comprises base stations (e.g., NodeBs, eNodeBs, gNBs) that create cellular coverage areas (cells) for wireless communication with User Equipment (UE). The Core Network provides switching, routing, subscriber management, authentication, and connectivity to external packet data networks like the internet.

From a technical standpoint, TN operation relies on a dense deployment of terrestrial base stations interconnected via a backhaul network (microwave, fiber, or cable). Radio communication uses licensed spectrum bands below 6 GHz (Frequency Range 1) and, in later generations, millimeter-wave bands above 24 GHz (Frequency Range 2). Key protocols and interfaces defined in 3GPP specs—such as the S1 interface between LTE RAN and core, or the NG interface in 5G—govern the communication between these terrestrial elements. Mobility management in a TN is primarily designed for handovers between terrestrial cells, managing interference in a dense, planned network topology, and providing low-latency access due to the short distances between UEs and base stations.

The role of the TN concept has evolved with the introduction of Non-Terrestrial Networks (NTN). In this context, 'TN' serves as the baseline reference model. When discussing integrated access backhaul (IAB), network slicing, or multi-connectivity, the assumptions and performance characteristics are often rooted in the terrestrial paradigm. For example, a TN typically assumes propagation delays of less than a few milliseconds, high reliability due to controlled interference environments, and the ability to deploy network functions in physically secure, powered locations. Understanding the TN is therefore prerequisite to understanding the enhancements and challenges introduced when extending coverage via satellites (NTN), as many NTN solutions aim to emulate or interwork seamlessly with the existing terrestrial network infrastructure and protocols.

Purpose & Motivation

The term 'Terrestrial Network' has been a foundational concept since the inception of cellular standards, but its explicit definition and contrastive use gained critical importance with the standardization of Non-Terrestrial Networks (NTN) in 3GPP Release 15 and beyond. Historically, all cellular networks were terrestrial by necessity, so the term was implicit. However, as the industry explored the integration of satellites (GEO, MEO, LEO) and High-Altitude Platform Stations (HAPS) to provide global coverage, complement 5G services, and serve Internet of Things (IoT) applications in remote areas, a clear distinction was needed.

This formal delineation solves the problem of ambiguity in technical specifications. It establishes a clear baseline architecture, channel model, protocol behavior, and performance expectation against which NTN extensions and adaptations can be defined. For instance, NTN scenarios must address challenges like very long propagation delays (hundreds of milliseconds), high Doppler shifts, and intermittent visibility that are absent in a typical TN. By defining 'TN', 3GPP creates a reference point, allowing standards to specify which TN procedures remain valid for NTN, which need modification (e.g., timing advance, handover, random access), and which new procedures are required. This enables the development of hybrid networks where a UE can seamlessly connect to either a terrestrial gNB or a satellite-based gNB, with the core network managing the integration. The motivation is to create a unified, global standard that supports ubiquitous connectivity, leveraging the high capacity and low latency of dense TNs where economically feasible, and the expansive coverage of NTNs where terrestrial deployment is impractical.